Thermal infrared radiation (IR) is emitted from all objects as a function of their temperature. IR imaging systems are able to detect thermal signatures and identify target objects by analyzing the heat and profile emitted. The mid-wave and long-wave IR detectors used in IR imaging systems are typically housed in vacuum enclosures, commonly referred to as dewars, and cooled to cryogenic temperatures to improve target detectability and lower signal to noise ratios.
A conventional IR imaging system 100 employing a typical dewar system 102 to cryogenically cool an infrared detector 104 is depicted in FIG. 1. The infrared detector 104 is mounted on a substrate 106 attached to a cold stem 108 of the dewar system 102. The cold stem 108 houses the refrigeration portion of a stirling cycle refrigerator which cools and maintains the detector 104 at cryogenic operating temperatures. The detector 104 is typically mounted within a coldshield 110 that is housed within a vacuum enclosure 112 of the dewar system 102. The vacuum enclosure 112 includes a window 114 attached to the top 116 of the vacuum enclosure 112 that allows the detector 104 to receive radiation signals external to the vacuum enclosure 112. The optics system of the IR imaging system 100 may be incorporated in the window 114 to the vacuum enclosure 112 or be positioned relative to the window 114 in the external housing (not shown in FIG. 1) of the IR imaging system 100.
The coldshield 110 typically includes a fixed aperture 116 that essentially forms the f-stop for the optics system of the IR while also serving as a radiation shield for the detector. Some conventional IR imaging systems are capable of switching between narrow and wide field of view window or optics system (e.g., window 114 or the optics system disposed in the external housing of the IR imaging system 100) to view various target scenes, which requires two different coldshield aperture sizes to effectively match the optics system. A large family of dewars with coldshields of different aperture sizes is currently needed to accommodate the broad range of IR camera system designs. Hence, the need arises for a dewar to have a single coldshield with a variable aperture assembly having two or more apertures that may be switched on command to accommodate the various optical systems that may be employed in an IR imaging system. Moreover, there is a need for an aperture actuator or control means that does not generate a significant amount of heat within the vacuum enclosure when powered on to drive the variable aperture assembly.
U.S. Pat. No. 7,157,706 to Gat et al. discloses variable aperture assemblies (each generally referenced as 122 in FIG. 1) for use in an IR camera having a dewar system 200 with a detector 104 mounted in a coldshield 110 that is enclosed in a vacuum chamber 112 as shown in FIG. 1. However, each non-magnetic driven variable aperture assembly 122 disclosed by Gat requires modifying the vacuum chamber 112 wall to either (1) add an external aperture control means 120, such as a worm gear system to drive a worm gear attached to the variable aperture assembly 122, or (2) to accommodate a piezoelectric motor aperture control means that directly contacts a friction surface of an outer drive ring of the variable aperture assembly 122. These conventional variable aperture assemblies and corresponding aperture control means are known to be extremely large in size (requiring significant space within the vacuum chamber or within the external housing of the IR imaging system) and require significant force and travel to control the size of the variable or swappable aperture to be used.
Accordingly, there is a need for an improved variable aperture assembly and aperture actuator assembly that overcomes the problems noted above and others previously experienced for implementing a variable aperture and actuator within a dewar system of a cooled IR imaging system or camera.